An Organic Light Emitting Diode (OLED) has various advantages over a Thin Film Transistor-Liquid Crystal Display (TFT-LCD), such as lower voltage requirement for driving, slim shape, wide viewing angle, faster response time, etc. Furthermore, the OLED provides equivalent or superior image quality, especially in small and medium sizes, to the TFT-LCD and is produced via a simple manufacturing process which leads to a competitive price as compared to other types of display devices. With such advantages, it has been spotlighted as a next step in display technology.
The OLED includes a lower substrate where an Indium Tin Oxide (ITO) transparent electrode pattern is formed as an anode on a transparent glass substrate, an upper substrate where a metal electrode is formed as a cathode on a substrate, and an organic light emitting material disposed between the upper and lower substrates. With this configuration, when voltage is applied between the transparent electrode and the metal electrode, current flows through the organic light emitting material, so that the OLED emits light.
Organic light emitting materials used for OLEDs were first developed in 1987 by the Eastman Kodak Company using small molecules such as aromatic diamines and an aluminum complex as a light emitting layer material (Applied Physics Letters, Vol. 51, p 913, 1987). Also, OLEDs using such materials were first reported as a device with practical performance by C. W. Tang, et al. [Applied Physics Letters, Vol. 51, No. 12, pp 913-915, 1987].
Those documents disclose an organic light emitting layer which has a stacked structure of a diamine derivative thin film (hole transport layer) and an Alq3(tris(8-hydroxy-quinolate)aluminum) thin film (electron transportable light emitting layer).
The foregoing materials for the light emitting layer are all polymers which are soluble in common solvents, unlike low molecular weight light emitting materials, and can form a light emitting layer by coating methods.
Recently, a variety of polymeric light emitting materials have been proposed (e.g. Advanced Materials, Vol. 12, pp 1737-1750, 2000).
In the case of manufacturing a device using the polymeric light emitting material by coating, the polymeric light emitting material provides merits in that manufacturing processes can be simplified and in that large scale devices can be easily made, unlike when low molecular vacuum deposition is employed.
When an electric field is applied to an OLED such that holes and electrons are injected from an anode and a cathode, the holes and electrons are recombined into excitons in a light emitting layer.
Then, the excitons emit light when returning to the ground state.
Light emission mechanisms can be divided into two categories: fluorescence using singlet excitons and phosphorescence using triplet excitons.
Recently, reports have been made that a phosphorescent material as well as a fluorescent material can be used as a light emitting material for an OLED (by D. F. O'Brien et al., Applied Physics Letters, 74 (3), pp 442-444, 1999; M. A. Baldo et al., Applied Physics Letters, 75 (1), pp 4-6, 1999). Phosphorescence is based on the following mechanism: after transition of electrons from the ground state to the excited state, non-emissive transition of singlet excitons to triplet excitons occurs through intersystem crossing, and then, transition of triplet excitons to the ground state occurs while emitting light.
Here, since the triplet excitons are spin-forbidden directly to the ground state in transition, they undergo transition to the ground state after flipping of electron spins. Thus, phosphorescence has a longer lifetime (emission time) than fluorescence.
That is, fluorescence has an emission duration of just several nano seconds, whereas phosphorescence has a relatively long duration of several micro seconds.
In view of quantum mechanics, when holes from the anode and electrons from the cathode are recombined into the excitons in an OLED, the generation ratio of singlet to triplet is 1:3, which means three times as many triplet excitons are generated as singlet electrons.
Hence, in fluorescence, 25% of the excitons are in an excited singlet state (75% in a triplet state), thereby limiting the emission efficiency. In phosphorescence, however, 75% of the excitons in the triplet state can be used along with 25% of the excitons in the excited singlet state, which enables 100% internal quantum efficiency to be obtained in theory.
Thus, a phosphorescent material can provide about three times higher emission efficiency than a fluorescent material in practical use.
For the OLED having the structure described above, a luminescent pigment (dopant) can be added to the light emitting layer (host) to improve efficiency and stability of light emission.
In this case, the efficiency and performance of the light emitting device can vary depending on what type of host is used for the light emitting layer. As proposed from studies on the light emitting layer (host), examples of an monomolecular organic host material include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorine, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene.
A polymer host can also make an idential function through synthesis of polymers comprising the molecules as proposed above as examples.
Although advances have been achieved in the field of OLED technology, there is a need of further improvements in emission efficiency, color purity, and electrical stability.
Moreover, the current advances in technology are insufficient to realize a large-scale light emitting device.
Thus, there is still a need of a polymeric light emitting material that can solve the problems of the conventional materials as described above.